1. Field of the Invention
This invention relates to integrated circuit power supply systems and, more particularly to supply systems that deliver more than one supply voltage.
2. Description of Related Art
In integrated circuit systems, development of CMOS process technology has allowed for higher component density and more device speed with lower power consumption. Reduction of power consumption is achieved mainly by using a lower supply voltage. However, diverse evolutionary paths of different modules (which are implemented as different integrated circuit) result in circuits with dissimilar requirements and characteristics. Some core ICs, like microprocessors, memory etc., are primarily implemented in submicron process to exploit technological advances while some peripheral modules are still fabricated in micron process due to cost reason. As a result, multiple supply lines are required to power various modules within a system. In such a multi-supply system, use of power management modules and power sequencers become critical to synchronize and monitor the different supply lines and hence, to avoid possible malfunction.
Within a power management (or supervisor) IC, a maximum supply selector circuit is a typical requirement. The maximum supply selector circuit provides power to the logic circuits of the power management IC even with presence of potential at only one supply line. To reduce the area of the power supervisor IC, some circuits can be shared for both the power supply monitoring paths. Bias, reference generator and all the internal digital logic circuits can be shared. The output buffers which generate the reset pulses and the supply monitoring potential dividers must have their respective voltage supplies. The power supply for the shared circuits should be the maximum of the two main supplies. A typical technique for implementing this requirement is to connect each power supply to the anode of a respective Schottky diode, the cathodes being connected to a supply output node. Thus the output voltage is the maximum of the two power supplies, minus the forward voltage drop of approx. 0.3 vdc.
However, in some IC manufacturing processes, it is difficult or not feasible to form Schottky diodes. And, for a low voltage power supply, the diode forward voltage drop may constitute a significant fraction of the supply voltage, and leakage may comprise a threat to the charge of a battery supply. These circumstances in the prior art have created a need for a circuit to select the highest power supply for an IC system, without using Schottky diodes.